It is known for some time that antisense phosphorothioate complementary to the messenger RNA of the HIV-1 rev gene inhibited the cytopathic effect of the virus in chronically infected H9 cells. The availability of these oligonucleotide analogues is thus critical for biomedical investigations. Our laboratory has recently reported the use of 3H-1,2-benzodithiole-3- one 1,1-dioxide as a sulfur-transfer reagent in the synthesis of oligodeoxyribonucleoside phosphorothioates. To promote the accessibility of the sulfur-transfer reagent, Judith B. regan and the P.I. have scaled up and improved the preparation of the reagent. This procedure has been published in Organic Preparations and Procedures International. Oligonucleoside phosphorothioates are inherently resistant to nucleases and are therefore difficult to characterize. One approach to the facile characterization of oligonucleoside phosphorothioates would be to convert them to natural oligonucleotides via oxidative desulfurization. Our laboratory has invested considerable efforts in screening various oxidants for the solid-phase conversion of oligonucleoside phosphorothioate triesters to the corresponding phosphotriesters. It has been found that a 0.05 M solution of monoperoxyphthalic acid in glacial acetic acid was ideal for the desulfurization of phosphorothioate oligomers. However, this reagent caused the modification of the cytosine ring. The search for new oxidants for the desulfurization of phosphorothioate oligomer was abandoned. In addition, the conversion of phosphorothioate oligomers to phosphoramidate oligonucleotides, another class of nuclease-resistant oligonucleotides, has been investigated but has not led to a reagent capable of performing a clean and rapid conversion.